To be able to have a certain rise/fall time (10/90) perecent  measurement a minimum bandwidth must be meet.

The general rules:   T(second)=0.35/BW(Hz)

T=Rise / Fall Time

BW=Bandwidth

 BW           T       
MHz          ns
10             35.000
20             17.500
30             11.667
40             8.750
50             7.000
60             5.833
70             5.000
80             4.375
90             3.889
100           3.500
200           1.750
300           1.167
400           0.875
500           0.700
600           0.583
700           0.500
800           0.438
900           0.389

GHz        psec
1            350.00
2            175.00
3            116.67
4            87.50
5            70.00
6            58.33
7            50.00
8            43.75
9            38.89
10          35.000
20          17.500
30          11.667
40          8.750
50          7.000
60          5.833
70          5.000
80          4.375
90          3.889
100        3.500